Shrinkage takes place in elastomers when polymers and catalysts react. Under normal processing conditions, shrinkage rates remain at 1-2%. Warping will occur with excessive shrinking which can lead to cracking – these cracks can grow into large surface cracks.

Causes of Shrinkage

Mismatched Resin & Mold Temperatures

The urethane reaction is exothermic which means once the material mixture starts to react the blend will heat up. Differences between the mixture and mold temperature will cause the material to cure at difference speeds depending on which interface the material is touching. This dissimilar cure speed leads stresses on the material. As the mixture changes from liquid to solid, it’s crucial that the two temperatures remain in balance. Gelling reactant will shrink if the mold temperatures are too low, often leading to cracking. When the temperatures are too high, there can be warping.

Localized Temperature Disparities

Internal stresses can occur due to localized hot and cold spots in the mold. This often leads to shrinkage, cracking and warping.

Solutions

Exotherm & Mold Temperatures Should Be Balanced

Peak exotherms are the highest reaction temperatures reached by mixtures and each polyurethane system has a different peak exotherm. Mixtures of a larger mass typically have a higher peak exotherm than smaller masses. The goal is to have the mold temperatures balanced within ±5°C of the peak exotherm.

Thoroughly Heat the Mold

Air in the oven must circulate so that the temperature is consistent throughout. Make sure that the mold remains in the oven until there is a uniform temperature.

Keep an Eye on Raw Material Temperatures

When degassed and mixed, raw materials tend to lose heat. They can also lose heat when sitting before mixing which means they need to be reheated so the correct temperature is maintained. Cracking can occur if the temperature is too high. It’s advised to use lower system temperatures and high mold temperatures.

If you need custom urethane adhesives and encapsulants, contact Resin Design at 781.935.3133. Our elite team of Polymer Scientists have years of experience designing and developing adhesives and encapsulants for a wide number of industries including aerospace, automotive and electronics. Entex 52101 and Entex 53060 are both made with aliphatic isocyanate backbones. Urethanes made from this chemistry are slower to cure, and therefore reach lower peak exothermic temperatures during cure which minimizes shrinkage. Additional benefits of using urethanes based on aliphatic structures are clear final products and UV stability.

Causes of Voids

Before de-molding, there could potentially be huge voids in the part or on the surface. The voids are visible with the naked eye and they occur at thin-walled sections.

Pot Life is Too Short – Air can be trapped for a short period of time when the batch is poured into the mold and displaces the air. Air then moves to the upper surface of the part. This process can take some time, and when the pot life is too short, the air bubbles don’t have reach the surface to self-release.

Mold Leaks – When mold leaks into the adhesive a void can easily be formed within this area. This typically occurs when the system has an extended pot life or a low viscosity.

Air Entrapment – Air is displaced from the mold when the batch is poured into the mold. A void will form when the air is trapped in different cavities and there are no mechanisms to allow the air to escape. This typically occurs in thin-walled areas when the viscosity is too high.

How to Fix These Voids

Mold & Vents Should be Redesigned – Since large voids occur when there isn’t proper ventilation, a new mold design may be necessary to improve ventilation.

Utilize Low Viscosity Urethanes – Air can easily escape when a low viscosity material is used. A system with an extended pot life and lower viscosity can decrease the void formation.

Reduce Splashing – Specifically when using large rollers, it’s best to tilt the mold before filling. You can also limit the amount of splashing when you pour down the side of the mold.

Use a Degassing Agent – The surface tension of the system can be reduced when a degassing aid is used. This allows air to escape with more ease. This will significantly reduce the formation of voids when higher viscosity materials are used.

Contact Resin Designs for Urethane Adhesives & Encapsulants

Resin Designs offers a range of products for various industries including aerospace, automotive and electronics. Entex 53051 is a clear, rigid polycarbonate bonder and encapsulant. This urethane is formulated, by use of two of the methods mentioned above, to minimize void generation. This material is low in viscosity, 400-900 cps mix viscosity, and contains a degassing agent which reduces surface tension allowing trapped bubbles to escape.

Urethanes are commonly used to encapsulate electronic circuits because of their moisture resistance and other positive attributes. These benefits are important, however sometimes issues arise that must be remedied immediately to achieve the best results.

Bubbles Appearing in Parts

On occasion, small bubbles do become visible throughout the parts and may not be localized.

Causes of Bubbles

Moisture Contamination of Components

Moisture reacts with the isocyanate in a polymer, producing carbon dioxide gas. This carbon dioxide gas accrues as small bubbles. Moisture can come from a variety of sources such as the prepolymer, catalyst, adhesive and primer. When you have moisture contamination of either the adhesive or primer, bubbles are visible only at the bond line.

Moisture Contamination of Mold

Molds that aren’t heated for an extended period over the boiling point of water may experience atmospheric moisture on the surface. In humid atmospheres, molds can have water condensation if they are cooled and not reheated. When water-based mold release agents are applied improperly, this can lead to water on the mold. When this occurs, bubbles typically appear on the surface.

Insufficient Degassing

When the prepolymer isn’t degassed before mixing AND the batch isn’t degassed before pouring, small bubbles will form.

Incorrect Pouring Technique

If there is a lot of splashing while pouring, bubbles will form in the batch. This is a major problem when it comes to low-viscosity systems where splashing occurs frequently.

Seal in the Processing Machine is Not Working

Degassing happens in the component storage tanks and when using a meter-mix dispensing machine, components are pumped from the storage tanks to the mixing head. Since they are pumped directly into the mixing head, it’s highly unlikely that air will end up into the mixture.

Fixing Air Bubbles

Place Materials Under Dry Nitrogen

Atmospheric moisture is much less likely to be present when nitrogen gas replaces the air in container holding the material. If you can’t get a hold of nitrogen, use a heat gun or hairdryer for roughly 10 seconds prior to putting the lid on.

Degas Components

In the components and the mixed system there could be entrained air so it’s crucial to degas components prior to and after mixing. Moisture can also lead to foaming so again, degas the raw materials.

Heat Mold Efficiently

Any moisture that has condensed on the mold will be removed when the mold is heated above the boiling point of water.

About Resin Designs Urethanes

Resin Designs creates several custom products including the Nexus 52101 which is ideal for automotives, bonding plastics, encapsulants and potting. The Entex 51011 is both soft and compliant, and suitable for high vibration environments. Fill out our contact form to get in touch with us or give us a call at 781-935-3133.

Resin Designs creates and designs custom epoxies for a variety of industries. These products provide superior adhesion to metals, ceramics, rubbers, glass and plastics. Below are several issues face when working with epoxies, as well as some tips on how to fix them.

Issue #1: The epoxy mixture has not cured once the cured time has passed.

Origin of Problem #1: Improper ratio of hardener to resin: The first step is to remove the uncured epoxy. Do not try to apply additional material over the epoxy. Make sure you are using the proper number of pump strokes and avoid using extra hardener for a faster cure. Extra hardener will lead to a different cured properties.

Origin of Problem #2: Low temperature: In colder weather, be sure to allow extra curing time since the reaction will occur slower. Adding heat will allow the chemical reaction to be maintained or even sped up. If your application required low temperature curing make sure to utilize a hardener cures at low temperatures.

Origin of Problem #3: Improper mixing: Again, remove the epoxy in this situation and don’t let new material cover the epoxy. Resin and hardener should be mixed together so there are no resin-rich or hardener-rich areas. If mixing manually a different static mixer might be necessary to allow homogeneous mixing. Diagnosing improper mixing is easy as the epoxy cure properties will not be consistent throughout the whole polymer.

Origin of Problem #4: Wrong products: After removing the epoxy, check for the correct resin and hardener. Hardeners and Resins are not interchangeable unless verified by the manufacturer. The resin won’t cure correctly with polyester catalysts or other brands of hardener.

Origin of Problem #5: Size of adhesive cured: The speed at which epoxies cure is directly correlated with the mass. If the curing adhesive is lower in mass than the described mass for cure time on a data sheet the gel and cure time will be longer. Some epoxy reactions may even stall if the mass is too small.

Issue #2: The bond has failed.

Origin of Problem #1: Epoxy wicked into porous surfaces, causing a void at the joint: First wet the bonding surface and then apply a thickened epoxy. For the porous surfaces, re-wet these and end grain.

Origin of Problem #2: Bond surface is contaminated: The surface should be cleaned and sanded, especially wood surfaces.

Origin of Problem #3: Bond area is too small for the load on the joint: Add fillets, bonded fasteners and scarf joints to increase the bonding area.

Origin of Problem #4: Clamping pressure squeezes epoxy out of the joint: Only a small amount of epoxy should be squeezed from the joint.

Issue #3: Epoxy got hot and cured too fast.

Origin of problem #1: Epoxy mass is too large: For batches of epoxies that are too large, mix smaller patches. The mixture should be transferred to a container with more surface area once mixed.

Origin of problem #2: Applications that are too thick: Create several thin layers for areas that are too thick.

Solution: Due to air (specifically moisture) and sunlight, hardeners tend to darken over time and become more yellow. This does not necessarily mean that the hardener is bad, but a requalification will be necessary.

Issue #6: The epoxy resin is taking too long to cure.

Solution: Once the epoxy resin has been applied, you can heat the room that the project is in.

Issue #7: I don’t know how to clean the cured epoxy resin.

Solution: Cured epoxy systems are quite chemical resistant so they should be removed with an epoxy-type paint stripper. This paint stripper must contain methylene chloride. Epoxy resins and hardeners that are uncured should be cleaned with ketones, lacquer thinner and alcohol.

Issue #8: Materials won’t bond together with epoxy resin adhesives.

Solution: Wood, aluminum and glass bond with epoxy resin adhesives. However, Teflon, polyethylene and polypropylene and nylon will not bond. Materials that are not similar bond well together with epoxy adhesives.

Issue #9: The formation of bubbles on the cured epoxy.

Solution: Cover the bubbles with peel-ply, release fabric or thick plastic. Bubbles typically form from epoxies curing too quickly. Try to cure at low temperatures or use a low temperature curing epoxy will minimize bubble generation.

About Resin Designs Epoxies

Resin Designs makes structural, non-conductive epoxy adhesives that don’t melt when exposed to heat. Our epoxies are rigid and have great chemical resistance. They’ll also cure at room temperature and will cure faster when heat is applied. Check out some of our custom epoxies, including the Nexus 84301 and Entex 84251.

UV curing adhesives are extremely convenient as they provide a way to rapidly cure a product in specific applications. There are two cure mechanisms that are extremely different from one another – cationic and free radical.

In regards to cationic UV cure products, such as Vividcure 86011, photo-generated acids catalyze polymerization of epoxides. Cationic UV systems have less shrinkage and will not experience oxygen inhibition.

See below for a few key points:

Cure Speed – Cure speed is dependent on the thickness, so expect a longer cure time if the layer is thick. You could possibly increase the cure speed time by increasing the movement of the molecules. This can be done by warming the material prior to the curing process.

Skin-Over – In order to increase depth of cure through thick layers, you may need to reduce the intensity of the lamp and cure for longer. When a thick layer’s surface is affected by high-intensity irradiation, only the top will polymerize creating a hard skin that will act as a barrier from further light penetration. This barrier inhibits the depth of cure of the material.

Moisture Sensitivity –Since the photo-initiators in cationic systems are acidic, moisture and bases will neutralize them. We highly recommend against curing cationic systems in humid environments. The cure speed, interestingly, can be increased in 30-60% relative humidity, but will stall at 70% or greater.

Substrate Considerations – As mentioned, bases will neutralize the acid catalysts that induce polymerization of the adhesive. Certain substrates, such as some metals and treated glass, are basic. Substrate poisoning will look like a small, uncured layer against the substrate. This problem can be solved by treating the substrate to neutralize the basic properties or switching out the substrate. Testing with normal curing parameters on a neutral substrate can help determine if the incomplete cure is due to skin-over or poisoning.

Mechanical Properties – While the mechanical properties of cationic cured systems are great, the reaction rate is slower than acrylate systems. The acids created by the photo-initiator during the radiation process will continue to crosslink the adhesive even after the radiation is removed. As such, we advise you measure the mechanical properties 24 hours after cure.

Post-Cure – Time can be reduced for a cationic system to reach full cure through the use of a thermal post-cure. The post-cure speeds up the reaction rate of polymerization.

Stress – As mentioned, compared to free radical systems, cationic systems have noticeably less shrinkage and therefore lower stress. Shrinkage and stress can be reduced by curing at a lower light intensity for a longer period of time.

Curing in seconds rather than minutes, free radical cure systems such as Vivid Cure 71141, are widely known for their rapid cure. Decomposition of the photo-initiator into free radicals by UV light starts a chain reaction curing mechanism that allows free radical cure systems to cure within seconds.

Take a look below at some important traits:

Cure Speed – Once the UV irradiation is complete, free radical cure systems can reach their full degree of cross-linking shortly after. This is due to their significantly quick cure speed. Cure speeds can be slowed down slightly by decreasing light intensity, and similarly increasing light intensity can slightly increase cure.

Oxygen Inhibition – One of the major downfall for free radical cure systems is oxygen inhibition. The growing-chains and photo-initiator radicals can be quenched by the presence of oxygen in the curing environment. This could cause short chain segments which lead to tacky surface layers as well as poor mechanical and physical traits. Using substrates on either side of the adhesive will isolate it from oxygen in the environment making them less susceptible to oxygen inhibition. Higher light intensity induces higher cure speeds that can decrease the effect of inhibition. This is because quicker chain formation permits the polymerization to proceed to completion faster than the quenching can happen. Light intensity can be modified by increasing the power from the light or decreasing the distance between the light and the adhesive.

Light Considerations – Over time lights will not function at 100% efficiency and will not be as bright. As such adhesives that used to cure in a process will start to fail. As such we highly recommend that companies, for quality purposes, measure and record light intensity and power when initially qualifying an adhesive. If power and intensity fall below threshold numbers you know that the light will need to be repaired or replaced.

Photo-initiators – A single photo-initiator cannot absorb all wavelengths of light. Specific photo- initiators that are used in adhesives are tailored towards either LED, metal halide, or mercury lights. We recommend understanding the spectral output of your light to make sure the emitted wavelengths line up with absorbance of the adhesive’s photo-initiator. Your light supplier can provide you with the spectral output of the light.

Post Cure – A thermal post cure will not have a negative effect on free radical systems nor will it benefit them.

Overview

In two of our previous articles, “Understanding UV Curing Adhesives” and “Cationic Epoxies- Advantages” we discussed the benefits of single component light curing adhesives. UV/light curing adhesives use energy from visible light or UV radiation to initiate polymerization. However, areas that are not exposed to radiation will not cure. This weakness has led to hybrid systems that allow for dark sectional curing through a secondary mechanism. Hybrid systems come in two forms, those that continue the use of the acrylate chemistry and those that incorporate new chemistries.

Homogeneous chemistry

Light radiation is not the only avenue for creating the free radicals that causes acrylate polymerization. Free radicals can be created using e-beam curing technology or heat.

Electron beam technology, otherwise known as E-beam, uses an electron accelerator to project an electron at its target. Unlike UV or light radiation, no photoiniator is required. This is because a free radical, used in acrylate curing mechanisms, provides an unpaired electron to initiate polymerization while the e-beam directly provides that unpaired electron. E-beam can penetrate certain substrates that would absorb light. The depth of penetration is directly linked to the energy used by the electron accelerator.

Heterogeneous chemistry

Isocyanates, which is one of the two functional parts of a urethane adhesive, can react with moisture to become polyureas as discussed in the article “Urethanes”. This technique can be used, and combined with, acrylate chemistry to allow dark sectional cure providing that there is moisture present. After the moisture reaction finishes the adhesive will have strong moisture resistance. As such the adhesive will become a barrier where the initial moisture penetration existed. Aside from moisture resistance and dark section curing, another major benefit to using a secondary moisture reaction is improved bond with difficult substrates.

Vividcure 76011 is one part dual cure UV Urethane. This product bonds well to difficult plastics such as santoprene, delrin, polypropylene, nylon, and PBT. It has had success many markets, including automotive.

In our previous blog “Urethanes” we discussed the advantages of urethane adhesives and applications when they are advantageous. One particular application that urethanes excel in is LED encapsulation. When high temperature resistance is not necessary, urethanes can be implemented in place of silicones. Urethanes work more efficiently in low temperature applications and are easily utilized for electronic packaging. Another major benefit when working with urethanes is that they protect stress sensitive electronic devices, as well as act as a barricade against water.

Polyurethanes are often utilized for their flexibility, shore hardness variances and short pot life. Polyurethanes are also perfect for delicate electronic components as they can act as shock absorbers. They are water resistant which makes them great resources for marine applications.

Polyurethanes & LEDs

LED lighting has become increasingly common, you may find them in illuminated furniture, surrounding cabinetry or concealed in room to produce indirect lighting. Two-component potting solutions are used to protect LEDs (polyurethane or silicone).

In order for an LED encapsulation to be working at an optimum performance, it should be potted with polyurethane or silicone products. This also protects the LED from mechanical damage, moisture or any other environmental disturbance. Two part potting systems allows the adhesive to be processed on fully automatic mixing machines.

Both space and time are saved when LEDs are casted with potting and silicone materials. The chosen sealant can greatly influence the properties of the emitted light. There will be a greater light dispersion and clear casting that does not interfere with the emitted light. Polyurethane-based optical casting protects from yellow which is generated from UV radiation by aliphatic isocyanate.

Urethane LED encapsulants can be used as a tool to control light emitting from the LED. These products can be made to be light diffusing if the application calls for it. See the video (or photos) below for the difference between a light diffusing and not diffusing urethane.

When designing LED lights and lighting systems, the casting material should first be determined. Resin Designs offers clear, ridged or flexible, urethane LED Encapsulants that can be light diffusing:

Understanding Urethanes

Urethanes offer an array of benefits including flexibility, water white clarity, low temperature functionality, and elongation as well as abrasion resistance. Due to these properties, urethanes are at an advantage compared to other adhesives in devices that are undergoing stress due to discrepancies of component composition.

Numerous urethanes consist of low glass transition temperatures so they have the ability to protect components in consumer electronics which operate at a range of -40C to 105C. Several urethanes can also be used at temperatures of -70C and others as high as 130C.

Chemists can manipulate urethane formulations to have either an extremely quick gel time at room temperature or a slow gel time with low exotherms. In fact, urethanes can be created to a range of hardness from soft gels to high Shore D.

Generally, urethane chemistry consists of a two-component system; one side is polyol and the other side is isocyanate. When moisture is in the air, the isocyanate will react so it’s encouraged to protect unused materials with a layer of nitrogen gas during storage. This will remove an air that is in the container.

Polyurethane Adhesives

Commonly referred to as “elastic adhesives,” polyurethane adhesives are glues which consist of urethane polymers; these polymers contain chemical based of isocyanate group. Typically you’ll find polyurethane adhesives in the following application:

Encapsulation/Potting/Sealing

Medical grade adhesives

Automotive applications

Plastic bonding

Application that require low temperature capabilities

Applications that require strong water/moisture resistance

Applications that are exposed to the outside elements (weather/sun radiation)

Cured by polyaddition reactions, polyurethane adhesives take on a reticulated structure. This structure can be ridged or flexible, elastomer like, dependent on the level of crosslinking.

Advantages of Polyurethane Adhesives

Due to the hydroxyl functionality of the polyol portion of polyurethane these adhesives are completely water resistant. Additionally, polyurethane adhesives can be formulated to be at a low viscosity, cure within seconds, and cure to a pliable state at room temperature conditions. As an adhesive, the cure time is determined by the length of time required before the adhesive has bonded two materials together.

Three Types of Polyurethane Adhesives

Two component polyurethane adhesives

One component polyurethane adhesives cured by heat

One component polyurethane adhesives cured by moisture

Polyurethane Adhesive Applications

Resin Designs has urethanes products that cover the majority of the described applications:

Entex 51021: A general purpose electronic encapsulate with success in underwater applications. Features include low Tg (-65 °C) and medium hardness (Shore A90).

Nexus 52101: Clear encapsulate and plastic bonder, especially to polycarbonate. Features include medium hardness (Shore 45D) and good elongation (170%). Axis M-100N is the medical grade version of this product.

Entex 53051: Crystal clear, UV stable, adhesive with success in lens coating. This material is more ridge (Shore D75) than other urethane products.

Nexus 51031: Strong adhesive bonder. Works well with thermoplastics, wood, metal, ceramic, and stone. This adhesive has a fast work life (3 minutes) and is ridged (Shore D80).

If you need a custom urethane adhesive please contact us. We look forward to work with you to help deliver the product you need.

UV display adhesives are commonly referred to as LOCA, otherwise known as liquid optically-clear adhesives. This specific type of adhesives is primarily used for the electronic industry, including products such as smartphones, touchpads, LCD screens and much more. Due to its rapid drying process, high transmittance and low shrinkage, LOCA has become an increasingly useful product among small businesses and larger corporations.

Several pieces of technology such as laptops, televisions, monitors and cell phones contain an air gap between the module and the cover glass. The touch interface will become universal within the next several years, leading manufacturers to enhance customer satisfaction through the strategies listed below.

Improvements in the Viewing Experience –Through both LOCA and A/R glass, the contrast ratio can increase by 400% in the sunlight.

Better Display Toughness – The falling ball impact resistance can increase by up to 3x depending on the gap size.

Longer Battery Life – The battery life can be significantly extended by minimizing the light loss caused by reflection. This allows the user to have a better viewing experience without the need to maximize backlight power.

Extended Display Product Life & Thinner Designs – LOCA increases the durability of a display to heat and moisture. Additionally, UV display adhesives allow for thinner designed electronics because they absorb the impact of a given load. Absorbance of impact is directly correlated with the adhesives modulus. The lower the modulus the more force is absorbed by the adhesive.

Applications Where LOCA is Utilized

Touch Panel Sensor

LOCA can be used to laminate two layers of ITO coated glass in a touch panel sensor, however, a majority of the time the two ITO layers can be placed on a single layer of glass. Some of the requirements include a low viscosity and durometer, high adhesion to glass, greater than 99% transmission, less than 1% haze, low shrinkage, yellowness (b*)<1, thermal shock and long term resistance to UV light.

Cover Lens Bonding

In order to sufficiently optimize the viewing experience, the air gap between the cover lens and touch panel sensor must be filled. Similar to a touch panel sensor assembly, this is a tight fitting application and contains the same key performance requirements.

Direct Bonding

Direct bonding allows the highest amount contrast and get the image close to the user. The LCD module is typically protected from degradation caused by expansion, contraction and impact due to the high bondline gaps. A high viscosity dam is typically required by higher gaps because they increase the edge definition. Key performance requirements include low viscosity, very low durometer, high adhesion to plastic and glass, greater than 99% transmission, less than 1% haze, yellowness (b*)<1, low shrinkage, long term resistance to UV light and thermal shock, and resistance to MURA.

General Requirements for LOCA

Some of the typical requirements for LOCAs are fast curing, optically clear, particle free, similar refractive index to glass, low shrinkage, low modulus, low hardness and high elongation.

There cannot be performance degradation after high temperature high humidity testing (65C/90%RH or 85C/85%RH), high temperature aging (85C or 95C), low temperature aging, UV aging, thermal cycling or thermal shock.

Why Low Modulus is Important for LOCA

A low modulus is one of the most critical elements when it comes to UV display adhesives. All UV acrylates reduce in size, whether minimally or otherwise, during cure. When you have a lower modulus, you will notice a greater reduction of MURA (fuzziness around the edge or center of a LCD screen). Mura can be caused by the stresses between the adhesive and substrate during shrinkage. The lower the modulus the less stress between the adhesive and the substrate. It’s important to note modulus become more important as screen size increases.

Choose Resin Designs for UV Display Adhesives

Resin Designs offers custom UV display adhesives for the aerospace, automotive, electronics, industrial and communications industries. Our Vivid Cure 70501 is a fast curing, water white, urethane acrylate that is specifically designed for LCD displays. This product offers a high amount of bond strength, excellent moisture and yellowing resistance. Additionally, the glass transition temperature of the Vivid Cure 70501 is low (-35 degrees C) which is quite unique considering a majority of LOCAs turn to into “rock” at lower temperatures. In fact, since other adhesives have higher glass transition temperatures, they do not have the same modulus properties at lower temperatures.